Condensation and mould

The condensation industry in the UK is a strange place. It produces miracle paints, magnetic dehumidifiers, anti-mould sprays, ionic air filters, and a small library of products promising to fix mould without you having to think about why it's there. Most of these don't work. The ones that do work fix only the symptom — a bit of black on the wall — without addressing the moisture imbalance that put the black there in the first place.

This is a building-physics problem dressed up as a product problem. Mould grows where moisture sits on a cold-enough surface for long enough. Reduce the moisture, warm the surface, or change the dwell time, and the mould stops. Don't reduce any of those, and you're painting over the symptom every six months.

Below is what's actually going on, what the quack products won't tell you, and the structured approach that actually solves a UK mould problem permanently. With caveats: some buildings are unfixable without major work, some households produce more moisture than the building can handle, and the right answer for your specific case may need a trained-eye assessment. But the framing here will tell you whether you're buying the right kind of help.

What's actually happening

Mould grows when three conditions are met: surface temperature low enough for sustained relative humidity above ~70%, moisture available, and time. Remove any one and growth stops. The classic UK mould patterns — corners of bedrooms, behind wardrobes on external walls, around window reveals, in north-facing utility rooms — are all places where one or more of these conditions is being created by the house's construction or the household's behaviour.

The diagnostic question is always: where is the moisture coming from, why is this surface cold enough to host condensation, and why isn't the moisture leaving fast enough? Answer those three questions and the fix becomes obvious. Don't answer them and any product you buy is hiding the problem rather than solving it.

Most UK condensation problems split into three categories: cold-surface condensation (windows, thermal bridges, behind furniture against external walls), high-moisture-load rooms (bathrooms, kitchens, drying laundry indoors), and ventilation failure (extract fans not working, trickle vents blocked, MVHR maintenance neglected). Each has a different fix.

Step 1: Measure the actual moisture load

Before any fix, measure. A £10 hygrometer in each room for two weeks tells you more than any product website. What you're looking for:

• Average relative humidity (RH) across the day. Comfortable range 40–55%. Above 60% sustained, you have moisture excess. Above 70% on cold surfaces, mould.
• Spike pattern after specific activities. Cooking, showering, drying laundry indoors — each produces a spike. The question is how high the spike goes and how long it takes to fall.
• Difference between rooms. A bathroom at 75% RH for 30 minutes after a shower is normal; one at 75% RH all morning is a problem.
• Surface temperature versus air temperature. Use an infrared thermometer on suspect walls. The cold-corner reading versus the room average tells you whether thermal bridging is creating condensation surface.

Two weeks of data costs £10 and an hour of attention, and tells you whether the problem is moisture-load-driven or surface-temperature-driven. Without it, you're guessing.

Step 2: Reduce the moisture sources

A typical UK family of four produces 8–12 litres of moisture per day through breathing, cooking, showering, washing, and (the big one) drying clothes indoors. If the building can't get rid of that moisture fast enough, it deposits on cold surfaces.

The high-yield interventions, in order:

1. Stop drying laundry indoors. A wet load contains 2–3 litres of water that ends up in the air over 24–48 hours. If you must dry indoors, use a vented or condenser dryer — never a clothes horse in a poorly ventilated room.
2. Use the bathroom and kitchen extract fans correctly. Run the fan during the shower and for 15–20 minutes after. Cooking with lids on pans reduces airborne moisture by ~40%.
3. Keep bathroom doors shut during and after showering. The moisture wants to migrate to the coolest surface in the house — usually a bedroom wall — if you let it.
4. Check that extract fans actually move air. A piece of tissue should pull firmly against the grille when running. If it doesn't, the duct's blocked, kinked, or disconnected.
5. Empty dehumidifiers and let them run during high-RH days rather than as the only solution.

None of these requires a product purchase. Most are behaviour or maintenance.

Step 3: Warm the cold surfaces

If moisture-load reduction isn't enough — and sometimes it isn't, particularly in cold corners or behind furniture — the next move is to warm the surface so condensation can't form on it.

Practical interventions:

• Move furniture 5–10cm away from external walls. The air gap lets the wall warm to room temperature, eliminating the condensation surface behind the sofa. Free.
• Insulate the cold surface. If a corner is consistently 4°C cooler than the room, internal wall insulation in that corner — even just an insulated wallboard or a properly designed thermal break at the floor-wall junction — solves the problem permanently.
• Address thermal bridges. The classic UK pattern is the concrete lintel above a window or the steel beam crossing an external wall — a bridge the surface temperature drops at sharply. These are diagnosable with an infrared camera and fixable with targeted insulation.
• Improve glazing. Single-glazed windows in winter run at 5–8°C surface temperature in moderate weather; condensation is inevitable. New double-glazing brings the surface to 12–15°C and the condensation stops.

The pattern: where mould is forming on a specific cold spot, surface temperature is the variable to address. Where it's forming generally, moisture load is the variable.

Step 4: Fix the ventilation

UK ventilation has been progressively failing as houses have been tightened up without ventilation strategy keeping pace. Trickle vents get blocked, extract fans fail silently, MVHR systems go years without filter changes. The result is houses that are tighter than designed and not ventilating well enough for the moisture load they contain.

The structured fix:

1. Audit every extract fan for actual extract rate. Tissue test, or a £30 anemometer for a real measurement. Replace anything failing.
2. Open trickle vents. Yes, they're a small heat-loss penalty. The trade is worth it. If you don't have trickle vents, fitting them retrospectively to windows is a £30–£60 per window job.
3. If you have MVHR, change the filters on schedule (every 6–12 months) and book a service every 2 years. A neglected MVHR is worse than no MVHR.
4. For chronically high-RH rooms, consider a continuous extract fan (MEV) running at low rate 24/7 with boost during high-moisture periods. £400–£900 fitted, and removes the underlying problem rather than chasing it.
5. For whole-house solutions, positive input ventilation (PIV) is a £600–£1,200 install that pressurises the house with filtered loft air, diluting moisture continuously. Effective for many UK houses with chronic condensation.

What the quack products do and don't do

A non-exhaustive list of products that don't work, and why:

• 'Anti-mould paint'. Contains a fungicide that kills surface mould for 6–24 months. Doesn't address the moisture or temperature reasons the mould was there. The mould comes back when the fungicide leaches.
• 'Damp-proofing injection'. Used to be sold as a fix for rising damp; the diagnosis was usually wrong (most 'rising damp' is actually condensation or external bridging). Injecting silicone into a wall that doesn't have rising damp wastes money.
• 'Mould-killing sprays'. Bleach kills surface mould. So does a £2 supermarket bleach. The expensive branded version contains the same active ingredient at a similar concentration.
• 'Magnetic / electronic damp-proofing'. No published evidence of effectiveness. Avoid.
• 'Ionic air purifiers' for mould. Reduce airborne spores marginally but don't address moisture — the wall still has the conditions to grow more mould.

The principle: any product that promises to fix mould without you reducing moisture or warming the surface is selling you a placebo. The real fixes are in the building physics.

When to bring in a damp surveyor

Some cases need professional assessment:

• Persistent damp in multiple rooms not corresponding to obvious moisture sources
• Damp at a specific height (suggests rising damp or a ground-level water source)
• Damp following heavy rain (suggests external water ingress)
• Recently purchased property with disclosed damp history
• Insurance or warranty claim where formal documentation is required

Choose an independent damp surveyor — not one tied to a remedial company. The PCA (Property Care Association) maintains a register, and the WRBS (Water-related Building Survey) qualifications are good signals. A proper survey costs £200–£500, produces a written report with measurements and recommendations, and is independent of any sales motive.

What you don't want is a 'free survey' from a damp-proofing company. The outcome is predictable.

Health implications worth knowing

The reason mould matters beyond the cosmetics is the health side, and this is where the public-health data has hardened in recent years. Sustained exposure to indoor mould is associated with respiratory irritation, exacerbated asthma, and in immunocompromised occupants, more serious infection. The 2023 Awaab's Law developments in the UK were a direct response to one of those tragic outcomes, and they've changed the landlord-tenant landscape for damp and mould. Owner-occupiers don't have the same legal hooks but the underlying health argument is the same.

The interventions that reduce mould also reduce a wider range of indoor air quality issues. Improved ventilation drops VOC concentrations, reduces dust mite populations, and lowers CO2 in occupied bedrooms — all measurable health benefits. Households with anyone managing asthma, COPD, or any respiratory sensitivity get a disproportionate health return from getting the moisture and ventilation right.

Where children or elderly residents are in the household, treat persistent damp as a higher-priority issue than the EPC-rating-uplift conversation suggests. The energy-bill argument is real, but the health argument is sometimes the bigger one. The fix is the same — moisture management, surface temperature, ventilation — but the urgency is greater.

UK mould and condensation is a building-physics problem, and the fix lives in moisture management, surface temperature, and ventilation — not in product purchases. The structured approach is: measure first, reduce moisture sources, warm cold surfaces, fix ventilation, and only then think about cosmetic remediation. The whole project for a typical UK house comes in under £1,000 in most cases, and addresses the cause rather than the symptom.

The quack-products industry exists because the building-physics fix isn't always convenient. Moving furniture, opening trickle vents, running extract fans for longer, investing in proper ventilation — none of these are as appealing as a £15 spray bottle. But the spray bottle won't solve the problem, and the proper fix will.